The invention relates to a voltage source converter with a line-side diode rectifier, a slim DC link and a power supply for the electronics of a load-side inverter of this converter.
Commercially available voltage source converters, also referred to as frequency converters, have converters on both the line side and the load side which are connected with one another on their respective DC side by a voltage DC link. The line-side inverter is implemented as a line-commutated converter, for example a diode rectifier, while the load-side converter is implemented as a self-commutated converter, for example a pulsed inverter. The voltage DC link includes one or more electrolytic capacitors. Moreover, a frequency converter includes a power supply for the electronics of the pulsed inverter which can be electrically connected, for example, by a voltage converter in parallel with the DC link capacitor. In the event of a power line failure, the power supply of the electronics of the pulsed inverter can then remain energized for a predetermined period of time. Charge to the DC link capacitor is supplied from the kinetic energy of the converter-powered motor. The DC link capacitor discharges after the kinetic energy is consumed.
If the DC link capacitor is essentially discharged before the line power is restored, then the line input of the converter experiences a high current flow. The magnitude of this charging current depends, on one hand, on the line impedance of the power line and, on the other hand, on the capacitance value of the DC link capacitor of the frequency converter. The high charging current can damage or destroy the diodes of the line-side converter, which may be prevented by using diodes with a highly I2T value. However, such diode rectifier would be over-engineered for the power required for continuous operation.
Other options for keeping the charging current low are disclosed in the publication “Netzrückwirkungen bei Frequenzumrichtern” (Line Feedback in Frequency Converters) by Karl Simon, printed in the German-language professional journal “etz”, Vol. 10, pp. 32 and 33, 2003. This publication describes various approaches for limiting line feedback in frequency converters. Line feedback in frequency converters is produced predominantly by the DC link capacitor by the current that recharges the capacitor during the voltage peak of a half period.
Conventional methods for reducing line feedback include the installation of additional chokes, either in the DC link or at the input of the frequency converter. In other words, the choke represents a current limiter that limits the recharging current. However, installing an additional choke disadvantageously adds to the cost and also requires a fair amount of space.
According to the aforementioned publication, this disadvantage can be overcome by using frequency converters with a so-called slim DC link. Such frequency converter has a significantly lower capacitance value of the DC link capacitor. The small capacitance value of the DC link capacitor allows for the use of foil capacitors instead of electrolytic capacitors. However, the DC link voltage then has a large ripple, and the average value of the DC link voltage is reduced. To prevent the ripple of the DC link voltage from affecting the quality of the rotation speed control of a converter-powered rotor, a modified control process is used for a pulse width modulator that takes the ripple into account. However, the motor then disadvantageously requires a more than 10% greater motor current under normal operating conditions, which increases the motor temperature. Because of the aforementioned disadvantages, a frequency converter with a slim DC link is preferably used in applications with pumps and fans.
Because of the smaller capacitance of the DC link capacitor, the power supply for the electronics of the pulsed inverter of the frequency converter that incorporates a slim DC link is connected to one phase of the supply line. However, this causes the power supply to fail in the event of line power interruptions, so that the operation of the frequency converter cannot be continued. If a frequency converter with a slim DC link is to stay operational even during a power failure, then the power supply must be buffered. This requires a special configuration of the power supply which also tends to be more costly.
Line power interruptions are more frequent in countries that have unstable power grids. However, these countries often require an improved service reliability, which can be provided by installing a choke in the frequency converters connected to such power grids. Because it is difficult in most cases to predict the line conditions ahead of time, substantial design work is required to dimension such choke. Otherwise, the frequency converter may fail.
If a frequency converter with a slim DC link is connected to an unstable power grid, failures of the frequency converter cannot be prevented even when using buffered power supplies and kinetic buffering.
It would therefore be desirable and advantageous to provide a voltage source converter which obviates prior art shortcomings and specifically has a higher service reliability even when connected to unstable power grids, without adding complexity and/or cost to its power line input.